Structure of Bituminous Coals: Evidence from Distribution of Hydrogen

STUDIES of the structure of vitrainous bituminous coals by X-ray diffraction1 and infra-red spectroscopic2,3 techniques have led to the paradoxical conclusion that although the greater part of the carbon (75 per cent) is ordered in aromatic systems, most of the hydrogen (for example, up to 80 per cent) in a coal containing 83 per cent carbon) is attached to the few carbon atoms not in aromatic systems. This paradox cannot be resolved by supposing the aromatic systems to be very large, or if small that they are heavily substituted by alkyl groups: the X-ray evidence denies the first supposition, and the second is inadmissible since the infra-red work shows that long alkyl chains are absent and the ratio of methyl to methylene groups is small. The average molecular weight4 is such that one molecule must contain a considerable number of these aromatic systems. Thus a molecule in coal apparently consists of a number of rather small aromatic systems, say 1–3 fused rings, highly substituted by aliphatic groupings that serve mainly to link together the ordered regions and mostly do not terminate in methyl groups.